1. Field of the Invention
This invention resides in asphalt technology and asphalt formulations, and also in waste management, particularly in connection with the disposal of scrap plastic.
2. Description of the Prior Art
Plastics are used in nearly every facet of modern life throughout the world. Food is wrapped in or protected by plastics; tools purchased at the local hardware store or warehouse store are encased in plastic; computers include numerous plastic components and structural features; plastic bags are used for handling trash; and even clothes contain significant amounts of plastics. The need for recycling plastics is great and continues to increase as the public becomes more conscious about environmental concerns and the depletion of natural resources.
There are many different types and varieties of plastics, and all are recyclable. A classification system has been established by the Society of Plastics Industry (SPI), as follows:
Class 1: Polyethylene terephthalate (PET) and polyethylene terephthalate ester (PETE), more commonly known as “polyester.” PETE fibers are made under the tradenames of DACRON® (E.I. duPont de Nemours & Co., Wilmington, Del., USA) and FORTREL® (Wellman, Inc., Fort Mill, S.C., USA). PETE film is known commonly as MYLAR® (E.I. duPont de Nemours & Co., Wilmington, Del., USA). Many food items are packaged in PETE and most of the transparent and colored 2-liter beverage bottles sold in grocery stores are made from PETE, with the exception of opaque bases which are typically made from High Density Polyethylene (HDPE).
Class 2: High density polyethylene (HDPE). HDPE is used for plastic milk bottles, water bottles, cosmetics containers, most plastic grocery bags, and trash bags.
Class 3: Polyvinyl chloride (PVC). Most of the PVC that is produced is used in the manufacture of plastic pipe and conduit. PVC is also used to produce vinyl siding, and vinyl window and door frames. PVC is also used to encase many items such as tools and toys, and is still used to produce plastic bottles. PVC may be made pliable with the addition of phthalate ester and then used to make raincoats, shower curtains, and rubber boots.
Class 4: Low density polyethylene (LDPE). LDPE is used in the manufacture of light weight plastic films, and for food and sandwich bags.
Class 5: Polypropylene (PP). Most PP is used in the production of auto and truck interiors such as door and instrument panels, although some is used in the food packaging. Another important use is in fibers for clothing and carpets.
Class 6: Polystyrene (PS). Polystyrene is used to produce styrofoam, which is used in packing, insulation, and food wraps. PS is also used for food containers that are clear, thin, and rigid such containers for salads and bakery goods. Many household items, including broom handles, television cases, computer cases, and dry cosmetic containers, are produced from PS.
Class 7 covers any plastics that do not fall in any of Classes 1 through 6. These include polytetrafluoroethylene (PTFE), polyurethane (PU), polycarbonates (PC), polyamides (PA) such as nylon, and the polyacrylamides and polymethacrylamides (PMA) used as an absorbent in diapers and potting soils.
The recycling of plastics generally requires that the sorting of plastic items by their classification, which is tedious and expensive. While many municipalities do sort discarded plastics, the plastics that are actually recycled are typically restricted to those of Class 1 (primarily PETE) and Class 2 (primarily HDPE). Class 3 (PVC), Class 4 (LDPE), Class 5 (PP), Class 6 (PS), and Class 7 (other) plastics are separated from the Class 1 and 2 plastics and sent for disposal in landfills. The need to recycle or reuse all of the various types of plastics in an inexpensive way, and to benefit from the reuse of the recycled plastics, thus remains.
Two general strategies for utilizing recycled plastic in asphalt have been disclosed in the prior art.
The first strategy is to incorporate the plastics into an asphalt binder or mastic. This approach finds its origin in the work of Charles H. McDonald which involved the incorporation of automotive tire rubber into asphalt. Patents disclosing this process are McDonald, C. H., U.S. Pat. No. 3,891,585 (“Elastomeric pavement repair composition for pavement failure and a method of making the same,” Jun. 24, 1975); Winters R. E., et al., U.S. Pat. No. 3,919,148 (“Pavement composition,” Nov. 11, 1975); McDonald, C. H., U.S. Pat. No. 4,018,730 (“Method for emulsifying asphalt-rubber paving material and a stable thixotropic emulsion of said material, Apr. 19, 1977); Nielsen, D. L., et al., U.S. Pat. No. 4,068,023 (“Rubberized asphalt paving composition and use thereof,” Jan. 10, 1978); and Woo, H. W., et al., U.S. Pat. No. 4,085,078 (“Method and apparatus for increasing contrast and resolution from low density film,” Jul. 12, 1977).
The concept was extended to recycled plastics by Haberl, P., in U.S. Pat. No. 3,853,800 (“Process For Preparing a Mixture Containing a Binder Material and Polyolefin,” Dec. 10, 1974), who used a high-molecular-weight polyolefin dispersed in a liquid polyolefin of low molecular weight. Other processes using polyolefins were disclosed by Hemersam, R., in U.S. Pat. No. 4,240,946 (“Method of preparing a bituminous binder and a construction material containing the same,” Dec. 23, 1980); Biegenzein, G., in U.S. Pat. No. 4,314,921 (“Method and apparatus for preparing a bituminous binder,” Feb. 9, 1982); Strommer, E., U.S. Pat. No. 4,988,747 (“Process for preparing a bituminous binder modified with plastics for building materials,” Jan. 29, 1991); and Strommer, E., U.S. Pat. No. 5,137,946 (“Process for preparing a bituminous binder modified with plastic for building materials,” Aug. 11, 1992). The specific use of low-density polyethylene is disclosed by Moran, L. E., in U.S. Pat. No. 4,868,233 (“Polyethylene modified asphalts,” Sep. 19, 1989). The use of oxygen-modified polyethylene is disclosed by Ho, K., et al., in U.S. Pat. No. 5,302,638 (“Asphalt/O-modified polyethylene,” Apr. 12, 1994). The use of scrap carpet backing containing polypropylene, styrene butadiene rubber, and calcium carbonate is disclosed by Kwok, W. L., et al., in U.S. Pat. No. 6,214,908 (“Composition containing novel modifier,” Apr. 10, 2001). Further use of polypropylene and polyethylene is disclosed by Forgac, J. M., et al., in U.S. Pat. No. 6,844,418 (“Waste plastic additive for asphalt,” Jan. 18, 2005). The plastics used in the patents listed in this paragraph are limited to those of Class 2, Class, 4 and Class 5, and the processes require separation of the plastics of these classes from those of other classes, which is expensive and tedious.
The second strategy is to use the plastic to supplement the mineral aggregate in hot mix asphalt paving compositions. Disclosures of this strategy appear in Brown, H. J., U.S. Pat. No. 3,852,046 (“Method for recycling waste plastics and products thereof,” Dec. 3, 1974); Baker, C., U.S. Pat. No. 4,018,722 (“Reclaimed plastic material,” Apr. 19, 1977); van den Berg, A. J., U.S. Pat. No. 4,028,293 (“Method for preparing materials containing minerals and synthetic substances, materials manufactured according to said method and objects consisting of said materials,” Jun. 7, 1977); Fishback, G. M., et al., U.S. Pat. No. 5,702,199 (“Plastic asphalt paving material and method of making same,” Dec. 30, 1997); and Berto, D., U.S. Pat. No. 6,030,572 (“Method for making a plastic aggregate,” Feb. 29, 2000). None of these patents, however, address processes that are applicable to scrap and recyclable plastic films that contain Class 2, Class 4, and Class 6 plastics, i.e., those that contain HDPE, LDPE, PP, and/or PS, which constitute a major portion of scrap municipal waste plastics. While these films can be shredded, the shreds cannot be successfully added to the hot mix paving compositions since they tend to clump and wind around the blades of the pugmills in batch plants and adhere to the mixing flights inside continuous drum mixer hot plants.
Adobe bricks are typically made from sand, clay, water, and fibrous material such as sticks, straw, or dung, forming a mixture that is shaped in frames into bricks and then dried in the sun for a period of time ranging from a few days to weeks. ASTM specifications for adobe bricks require that the soil meet certain particle size distributions, contain certain quantities of silt and clay, and have a plasticity index between 2 and 15 and an unconfined compressive strength of at least 300 pounds per square inch. The specifications further require that the brocks absorb no more than 2.5% by weight of water when only one side of the brick, with a water-proof coating applied, is exposed to water for seven days. Asphalt emulsions, pine tar, and tall oil pitch emulsions have been used in place of the water to achieve bricks with slightly higher unconfined compressive strengths and greater water resistance.